The present invention relates to a vehicle power steering system, and more specifically to a hydraulic vehicle power steering system which transmits a larger manual steering effort to the steered wheels at higher vehicle speeds than is transmitted at lower vehicle speeds.
A known rotary power steering directional control valve has an inner valve member (valve core) which is coaxial with and rotatable relative to an outer valve member (valve sleeve). To effect actuation of a power steering motor to turn steerable vehicle wheels, the inner valve member is rotated relative to the outer valve member for a few degrees against the bias of a torsion bar. Since actuation of the control valve is resisted by only the torsion bar, the resistance to actuation of the valve does not vary as a function of vehicle speed. The construction of this known power steering control valve is described in U.S. Pat. No. 4,276,812 issued Jul. 7, 1981 and entitled "Power Steering Valve and Method of Making the Same".
It is known to increase the resistance felt by an operator of a vehicle to actuation of a power steering system as vehicle speed increases by having the fluid pressure in a reaction chamber decrease as vehicle speed increases. The fluid pressure reaction chamber is provided to regulate the torque required to rotate an inner valve member relative to an outer valve member. Decreasing the fluid pressure in the reaction chamber increases the resistance to relative rotation between a pair of valve members which control flow to a power steering motor. U.S. Pat. No. 4,819,545 is one example of such a system.